/Engine/Source/Editor/UnrealEd/Public/SEditorViewport.h
Implement this to add an arbitrary set of toolbars or other overlays to the inside of the viewport.
virtual void PopulateViewportOverlays ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SOverlay](API\Runtime\SlateCore\Widgets\SOverlay) > Overlay )